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Full text of "Effects of roughness at high Reynolds numbers on the lift and drag characteristics of three thick airfoils"

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ACE No. IAH21 



S^ 



n 

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 



WARTIME REPORT 



ORIGINALLY ISSUED 

August 19*+^ as 
Adyance Ccaafldentlal Report IAH21 



THE EFFECTS OF EOIXJHKESS AT HIGH REIHOLDS NUMBEI^ 
ON THE LIFT AND IRAG CHARACTERISTICS 
OF THFtEE THICK AIRFOIIS 
By Frank T. Ab"bott, Jr. and Harold R. Turner, Jr. 

Langley Memnrlal Aeronautical Lalioratory 
Laneley Field, Va. 



''teSSSr.'SK'riKlfP 



NACA 



WASHINGTON 

NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of 
advance research results to an authorized group requiring them for the war effort. They were pre- 
viously held under a security status but are now unclassified. Some of these reports were not tech- 
nically edited. All have been reproduced without change in order to expedite general distribution. 



L . i;6 DOCUMENTS OEPARTMFN" 



NACA ACR No. 1J4H2I 

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 



ADVANCE CONFIDENTIAL REPORT 



THE EFFECTS OF ROUGHJIESS AT HIGH REYlv'OLDS NUMBERS 
Oil THE LIFT AND DRAG CHARACTERISTICS 
OF THREE THICI-: AIRFOILS 
By Frank T. Abbott, Jr. and Harold R. Turner, Jr. 

SUMMRY 



In connection with studies of airfoils applicable 
to largo high-speed aircraft, the effects of roughness 
on three 22-percent-thick airfoils were investigated. 
The tests were made over a r--.nge of Re.7nold3 nu;nber from 
about 6 to 26 X 136 for the airfoils s;!incth and with 
roughness strips applied to the surfaces. The results 
indicated that for the roughened iriodels the scale effect 
Was generall" favorable. 



INTRODTTCTION 



Previous tests in the NACA two-dimensional low- 
turbulence pressure tunnel of thick airfoils with 
roughened leading edges (reference 1) indicated that 
the lift and drag characteristics of the thicker wing 
sections, when accidentally roughened, would have an 
important bearing on the choice of sections for large 
high-speed airplanes. These tests were limited to 
Re'molds numbers much lower than the flight values for 
such airplanes hj the use of 2-foot-chord wooden models. 



The desirability of extending the tests to higher 
values of the Reynolds number was apparent, and an air- 
craft manufacturer submitted three J-foot-cliord models 
of heavy metal construction for this purpose. The three 
airfoil' sections v;ere: an NACA 63 (Ij.20)-Il22 -airfoil; an 
NACA 65(223 ) -1^22, a = 1,0 (anprox.) airfoil, where 
"(arprox,)" refers to a slight thickening near the 
trailing edge; and a 22-percent-thick Davis airfoil. 
These models were tested in the NACA two-dimensional 
low- turbulence -Dressure tunnel in order to obtain lift 



CONFIDENTIAL MCA ACR No. iJ^HZl 



and drag characteristics at Reynolds numbers up to approxi- 
mately 26 X 10° v;ith smooth surfaces, with roughness 
grains of various sizes on the leading edges, and in 
some cases with roughness strips at various chordwise 
positions. 



TEST METHODS 



All tests were conducted in the NACA two-dhnensional 
low-turbulence pressure tia:inel, which is characterized 
by an air stream of extremely lev/ turbulence. The models 
extended from wall to v/all of the rectangular ter.t sec- 
tion. Lift measurements were obtained by a manometer 
arrangement that integrated the lift reaction of the 
models on the floor and ceiling of the tunnel, and drag 
measurements were m^ade by the wake-survey method (refer- 
ence 2). A correction of aboi;t 2 percent v/as applied to 
the data for normal t\mnel-wall-constriction effects. 
Lift coefficients near m.aximum lift v;ere further corrected 
for additional t'onnel blocking that occurs v/hen the model 
is partially stalled. These additional corrections, 
derived from static-pressure mieasurements made along the 
floor and ceiling of the tumnel, varied from to about 
10 percent. Tests were m.ade at tunnel tank pressures 
from 50 pounds per square inch to I50 poijinds per square 
inch and, at all times, the tunnel airspeed was low 
enough to avoid compressibility effects. 

The airfoils submitted by the aircraft manufacturer 
had 56-inch chords, were of heavy metal construction, and 
were painted to give aerodynamlcally smooth surfaces. 
The two low-drag airfoils were tested first smooth, then 
with various sizes of roughness on the leading edge, and 
finally with 0.011 -inch roughness grains at one or more 
chordv/ise positions. The Davis airfoil was tested smooth 
and with roughness grains of two sizes on the leading edge. 

Tests were made of all three models, both smooth 
and rough, at Reynolds niimbers of approximately 6, 10, 
1I+, 20, and 26 x 10^. 

The roughness sizes of 0.002, O.OCli., and 0.011 inch 
represent the average size of the carborundimi grains 
used. The roughness was applied to the leading edge 

CONFIDENTIAL 



NACA ACR No. rJ+H21 CONFIDENTIAL 



by coating a strip fro?n 5. 50 to 5 '75 inches wide, sym.- 
raetrically spaced about the chord line at the leadirg 
edge, with thinned shellac and sprinkling with carbo- 
rmidutn grains until 5 to 10 percent of the area was 
covered v/ith grains. The roughness strips at 20 percent 
and 50 percent of the airfoil chord (0.2Cc and 0.5Cc) 
were similarly applied b^it were 0.5 inch wide wibh the 
forward edge of the strip at the specified chordwise 
location. 



RESULTG AND DISCUSSION 
NACA 63(1;.20 )-!,22 Airfoil 



The effects on the lift and drag characteristics of 
four sizes of roughness applied to the leading edge of 
the NACA 65 (l!-20 )-li.22 airfoil section at a Re^^molds number 

of 26 X 10° are shown in figure 1. Tb.e loss in maxirnum 
lift tended to be gradual with increasing roughness size, 
but the increase in drag coefficient in the low-drag 
range was not gradual. The application of shellac alone 
to the leading edge caused a large increase in drag coef- 
ficient in this range. The shellac, however, did not 
decrease the lift coefficient at which the drag increased 
sharply to extremely high values, whereas all other 
roughness sizes on the leading edge did. 

The effects of the 0. Oil-inch-grain roughness at 
various chordwise positions are shown in figure 2. There 
was no large detrimental effect on maximum lift •'onless 
the rough_ness was on the leading edge. This result is 
attributed to the fact that at ma:-cimum lift the shape of 
the pressure distribution causes transition on the upper 
surface to occur close to the leading edge. The effect 
of roughness, therefore, in the thick turbulent boundary 
layer downstream of the pressure peak would be expected 
to be small in comparison with the effect of roughness 
in the relatively thin boundary layer at the leading 
edge. The drag coefficients at low and moderate lift 
coefficients increased as the roughness was moved toward 
the leading edge, as _ would be expected from the accom- 
panying forv/ard movement of transition. The roughness 
strips at 0.20c and 0.50c, however, did not appreciably 
affect the value of the lift coefficient at which the 
drag increased to extremely high values. At these 

CONFIDENTIAL 



k CONPTDENTIAL NACA ACR No. li4.H21 

locations, the boundary layer cannot be laminar at such 
lift coefficients because of the shape cf the pressure 
distributions. 

The scale effect on the lift and drag characteristics 
of three sizes of roughness on the leading edge Is shown 
in figures 3 to 5« These plots show, in general, a 
gradual decrease in drag and an increase in raaxlmiim lift 
Y/lth increasing Re},Tiolds number - that is, the scale effect 
was considered favorable - for all three sizes of roughness. 



NACA 65(223)-l|22 (Modified) Airfoil 

Lift and drag characteristics of the NACA 65 (225 )-Ii.22 
(modified) airfoil are sho\vn in figure 6 for four model 
conditions; namely, 0.00l4.-inch-graln roughness on the 
leading edge, 0. Oil -inch- grain roughness on the leading 
edge, 0.011-inch-graln roughness at O.JOc, and smooth at 
Reynolds numbers of ll^. and 26 x 10°, The curves for the 
model in a sm.ooth condition are presented to show that 
this section had a gradual Increase in drag Vi^ith increasing 
Reynolds number - that is, the scale effect was con- 
sidered adverse - in the low-drag range. This result 
was probably caused by some slight surface irregularity'- 
v/hich, because of the small slopes of the favorable pres- 
sure gradients of this section, make it unusually sensi- 
tive to any surface defects and lonfairness. It is thought 
that lower drags than are shown for this section are 
possible, but NACA 65-serles airfoils (reference 2) which 
are preferable to the one tested are now available. 

The application of roughness to the leading edge of 
the NACA 65 (223 )-l4.22 (modified) airfoil seriously 
decreased the maximum lift and caixsed a large decrease 
in the lift coefficient at which the drag increased 
rapidly. The greater part of the drag increment attri- 
buted to the roughness grains was caused by the smallest 
roujjhness size tested. The roughness strip at 0.30c did 
not affect the maximum lift coefficient to any great 
extent, because the flow over the top surface of the 
airfoil at this high positive angle of attack had become 
turbulent much nearer the leading edge. 

The effects of 0.00i|-lnch-grain and 0.011-inch- 
graln rouglmesses applied to the leading edge at 

Reynolds numbers from 6 to 26 >: 10° are shown in fig- 
ures 7 3ind 8, respectively. The scale effect was 

CONFIDENTIAL 



TAG A ACR JIo. r4H21 CONFIDENTIAL 



generall:,'" favorabl9, especially in the case of the 
drag coefficients, hut became \^ery small at Re;^,'nolds 

mxmbers of 20 to 26 x 10". The increase with Reynolds 
mxnber- of the value of the lift coefficient at v/hich 
the drag coefficient increased sharply v/as especially 
notable . 



Davis Airfcil 

Lift and drag data for the Davis airfoil in the 
smooth condition and v/ith 0.002-inch"grain and 0.011-inch- 
grain roughnesses applied to the leading edge are pre- 
sented in figiire 9« A comparison of the lift and drag 
curves obtained for the smooth model with the curves 
obtained with roughness on the model shows that even the 
smaller (0.002-inch grain) rouglmess caused a loss in 
maxlmuin lift coefficient of arjproximately 0.1\., a slight 
decrease ^n lift-curve slope, and a large increase in 
drag throughout the range tested. 

Results ol' tests wltb roughness grains of 0.002 
and 0.011 inch on the leading edge at Reynolds numbers 

from 6 to 26 x IQ'^ are presented in figures 10 and 11, 
respectively. Scale effect on the drag coefficients 
was favorable for both sizes of roughness by.t became 
small at Reynolds numbers of 20 and 26 x 10'='. There 
was a small favorable scale effect on the maximum-lift 

valTies up to Re^molds numbers of 20 x 10*^ and small 
adverse scale effect for both sizes of roughness at 

Reynolds numbers from 20 to 26 x 10°, 



COMPARISON OP AIRFOIL SECTIONS 



The drag coefficients of the NACA 63(L23)-1|22 
airfoil section and the NACA 65 (225 )-i+.22 (modified) 
airfoil section vvlth roughness stri-os of 0.011-inch 



■^o 



grain at O.JOc are compared in figure 12. In this 
condition the extent of the laminar boundary layer 
should be the same for both sections at lift coeffi- 
cients corresponding to the lov/-drag range for the 
smooth airfoils. The drag coefficients were nearly 
the sajne for lift coefficients below about' 1.2; the 
differences shown are not considered greater than 

GONPIDENTI^.L 



f 



COin^IDEITTIAL NACA ACR ITo . iJ+HZl 



_ ossible variations for teats with roughness. Pig- 
v.ve 13 shows a similar conparison for the three airfoils 
tested with 0.011-inch-grain roughness on the loading 
edges. The NACA G3 ih-^-O ) -l\.2Z section was more resistant 
to separ'aticn when ro^Agh than the other two sections; 
that is, the lift coefficient at T,i'hlch the drag coeffi- 
cients rise sharply to very high values vms appreciably 
higher for this section than for the other sections 
tested. Numerous spanwise drag surveys ".vere made of 
the tliree models with roughnesj on the leading edges. 
These surveys showed that the HACA 63(ij-20)-I{-22 airfoil 
had no localized separation v.p to moderately high lift 
coefficients, that the XACa 65 (223 )-422 (.aodified) air- 
foil sho'ved marked local separacion at much lower lift 
coefficients, and that the Davis airfoil showed local 
separation at lift coefficients above approximately 0.8. 

The Gffects on the drag coefficient at a lift coef- 
ficient of O.L|. of various sizes of roughness on the 
leading ddge for the three airfoils tested are shown in 
figure ll|.» All throo airfoils had nearly the same drag 
coefficient whe:i rough and the drag increased very 
little with increasing rou.ghn3S3 size, A large increase 
occux'red, however, from the s-nooth condition to the 
smallest size of I'ougxir.ess . 

Both the NACA low-drag airfoils were affected 
by rougimesr less at the high Reynolds mx'n.bcrs than 
at the lov/er Reynolds numbers. This favorable scale 
effect v;ith the models in a rough coxadltion increased 
the lift coefficients at which the drag coefficients 
increased rapidly to extremely high values by nearly 0.I4. 
for the NA0A"65(225)-ii-22 (modified) section and 0,2' for 
tb.-e >LvCA •j3(l+20)-i|.22 section. The Davis airfoil shov/ed 
practically no favorable scale effect in this respect 
although th^^ effect on drag coefficient at lower lift 
coefficients was favorable. 



CONCLUSIONS 



Tests of an NACA 65('.|.20)-)^22 airfoil, an 
NACA 65(225 )-I|.22 (modified) airfoil, and a 22-percent- 
thlck Davis airfoil, all with roughness strips on the 
surfaces, indicated the following conclusions: 

1, In genera]-., the airfoils with rougliiiess strips 
shov/ed favorable scale effects over the Re^Tiolds number 

CONFIDENTIAL 



NACA ACR No. LI4.H2I G0NFID3ITTIAL 7 

range from 6 to 26 x 10°, This favorable scale effect 
was particularly effective on the MCA airfoils in 
increasing the lift cooff Icients at which the drag 
coefficleats increased sharply to very high values. 

2. At sraall and moderate lift coefficients, the 
drag coefficients for all the sections tested ;vith 
leading edges rough were nearly the sajne for the same 
roughness condition and Reynolds nuxrJoer, With roxigh- 
nsss strips at 5^ percent of the chord, the drag charac- 
teristics of the tv.'o NACA airfoils tested v;ere nearly 
the sanio except at the highest lift coefficient. 

5. Increasing the size of the routv;hiness grains 
applied to the leading edge pi'ogressively decreased 
the raaxir;:ura lift coefficients for the sizes tested, 
but the greater oart of the drag increment caused by 
the roughness occurred with the sr.iallest roughness tested, 

i}.. The order of merit of the three airfoils in 
pemitting high lift coefficients to be obtained without 
excessively high drag coefficients with the leading edges 
rough is as follows: the NACA 65 (i|.20) -14.22 airfoil, the 
NACA 65 { 225 ) -[(.22 (modified) airfoil, and the 22-percent- 
thick Davis airfoil. 

5. The naximum lift coefficients of the NACA air- 
foils tested were not affected to any great extent by 
roughness strips at 20 or JO percent of the chord back 
of the leading edge. 



Langlejr T.femorial Aeronautical Laboratory 

National Advisory Goir!:nittse for Aeronautics 
Langley Field, Va. 



REPZR3NGES 




to Separation. N/'GA G3, June l'^k.2. 



2, Abbott, Ira E., von Doenhoff, Albert E., and Stivers, 
Louis 3,, Jr.: Sur3Vlar^^ of Airfoil Data. NACA AGR 
No. L5CO5, l'9kb- 

GOlfPIDENTIAL 



Digitized by tlie Internet Arcliive 

in 2011 witli funding from 

University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation 



• 
http://www.arGhive.org/details/effectsofroughneOOIang 



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l[/NIVERSITY OF FLORIDA 



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UNIVERSITY OF FLORIDA 
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